Phenolic compounds and resins are synthetic materials that vary greatly in molecular structure. This variety allows for a multitude of applications for these compounds; for example, use as a curing agent and/or to prepare the corresponding epoxy, cyanate and/or allyl thermosettable monomers and resins. These curing agents and/or monomers can provide enhanced physical and/or mechanical properties to a cured composition, such as increased glass transition temperature (Tg). To achieve improved properties, however, would require the thermosettable monomer to have a high functionality (i.e., chemical groups available for cross linking). However, as the functionality increases in these monomers, so does their molecular weight, which increases the melt viscosity of the monomer and can lead to difficulties in using such monomers. Likewise high functionality in the thermosettable monomers can lead to excessive enthalpic cure energy. Excessive exothermicity on curing can damage parts, such as laminates, composites or castings, causing cracking or delamination.
One strategy for preparing thermosettable monomers is to convert a phenol, such as bisphenol A, into the corresponding cyanate. Bisphenol A dicyanate (cyanuric acid ester of 4,4′-isopropylidenediphenol) (BPA DCN) was the first dicyanate monomer offered commercially and still serves as an industry standard. While the homopolytriazine of BPA DCN provides a relatively high 275° C. Tg, numerous problems were encountered. These problems included incomplete curing, generation of excessive enthalpic cure energy, poor moisture resistance and brittleness leading to deficiency in certain mechanical properties, notably tensile elongation and fracture toughness. One attempted solution to the deficiencies employs BPA DCN as a reactive component in blends with other thermosettable monomers, notably epoxy resins and bis(maleimide)s. While the blends generally afford some improvement in processing and mechanical properties, Tg is also typically reduced. Furthermore, tradeoffs in mechanical property improvements exist. For example in cured blends of BPA DCN with a novolac epoxy resin, oxazolidinone structure from the co-polymerization can modestly enhance flexural strength, but at the expense of large (>50%) decreases in tensile strength [Mathew, et al., Journal of Applied Polymer Science, volume 74, pages 1675-1685 (1999)]. Another attempted solution, toughening with thermoplastic additives, required control over phase separation during the cyclotrimerization reaction to prevent deterioration of Tg. Recent publication by Reams, et al. (ACS online publications, ACS Applied Materials & Interfaces, Feb. 6, 2012, dx.doi.org/10.1021/am201413t) provides extensive investigation of complex moisture resistance problems associated with BPA DCN.
Another approach to improvement of thermoset cyanate resins uses a phenolic precursor with different chemical structure. One example is taught in published U.S. Patent Application Number 2011/0009559, where the diphenol, 1,1-bis(4-hydroxyphenyl)cyclododecane, is converted to the corresponding dicyanate. Homocyclotrimerization of this dicyanate gave a Tg of 202.1° C., which is a substantial reduction from the 275° C. Tg for homocyclotrimerized BPA DCN, but it also had enhanced thermal resistance, gave low moisture absorption, excellent dielectric properties and moderation of the enthalpic cure energy, without increasing the cure onset and end temperatures.
There is unmet need for a polycyanate capable of homocyclotrimerization to give a polytriazine with a Tg substantially higher than 300° C. Likewise there is a need for a polycyanate which can be blended with other cyanates such as BPA DCN and/or other thermosettable monomers such as bis(maleimide)s to produce copolytriazines or copolymers, respectively, with a Tg above 275° C. while improving curing profile (for example decrease onset to cure temperature and enthalpic cure energy).
Thermosets of cyanate resins of CDD polyphenols have now surprisingly been found to provide remarkable Tg's (>400° C.) and improved cure profile including rapid onset to cure and reduced cure enthalpy. Furthermore when used in blends with other thermosettable monomers, for example with BPA DCN or the bismaleimide of 4,4′-diaminodiphenylmethane, cyanate resins of CDD polyphenols may impart increased Tg, thermal stability and/or improved cure profile to the thermosets thereof. Improvements in one or more of these properties provide higher performance thermosets, useful in structural or electrical laminates and/or composites, multilayer electronic circuitry, integrated circuit packaging (such as “IC substrates”), filament windings, moldings, encapsulations, castings, composites for aerospace applications, adhesives, functional powder coatings and other protective coatings. The cured compositions described herein are particularly useful in the aerospace and electronics industries and may be used in the form of sheets, films, fibers or other shaped articles.